just check all your connections with some soapy water and a brush. If any of your co2 pipe work is leaking try wrapping some tight wire around the joints (push on joints only can be wired tight). If the leak is coming from the top of the bottle itself, then dont attempt to fix it! Take it back. You dont want any sudden venting of your cylinder through over tightening of the main shut off valve. If thats the problem it cant be fixed while its under pressure. If the main valve itself is leaking, again take the whole thing back.
If its coming from the reg to cylinder connection, turn it off, Inspect the reg to cylinder sealing face, make sure there is a suitable seal, like a washer or o ring, (unless its a swaged brass fitting.) then put it back on. Make sure your cylinder's off when you do this obviously. Be carefull not to overtighten, but it may need nipping up a bit.
Problem areas on the low pressure side, check, Bubble counters, Non return valves, external inline diffusors. Then finally check the hose for leaks.
Please be carefull, compressed cylinders of any sort are extremely dangerous just treat em with respect all will be well.
Regulators themselves rarely leak but you can fix that yourself by shutting off main valve removing, fixing leak etc. As long as its something simple like the stem, otherwise take that back as well, it will be under warranty.
Old regulators, can leak when the seals perish, for this reason in industry all regulators must be inspected or replaced on a regular basis.
Just think before you do anything, it may only be co2, but there is an awfull lot of potential energy trapped in that little bottle.
